phase D milestones 1+2: referral system + paid-access gate

Lays the billing-prep spine before Paddle lands in D.3.

D.1 — referrals
- users.referral_code: unique 8-char URL-safe code (alphabet excludes the
  ambiguous 0/O/1/I/L). Generated lazily on first /settings hit so existing
  accounts pick one up without a backfill migration.
- users.referred_by_user_id + new referrals audit table (referrer,
  referred, created_at, converted_at, credited_at). converted_at /
  credited_at stay null until D.3 fills them via the Paddle webhook.
- POST /login accepts ?ref=<code>; the code rides on the signed
  pending-verify cookie so it survives the GET → POST → /verify hop.
- /settings page: email, tier badge, referral code chip + invite link
  with one-click copy, pending/converted/active-credits stats grid.
  Settings nav link added to the top bar.

Reward shape: when the referred user makes their first paid Paddle
subscription, both they and the referrer get 50% off for 3 months.
(D.3 wires the actual credit application via the Paddle webhook.)

D.2 — paid-access gate
- users.credit_until: timestamp until which a free-tier account has
  paid-tier access. Null = no credit. Populated by admin CLI now and the
  D.3 webhook later.
- app.services.access exposes paid_status(user) → PaidStatus dataclass
  (active / source / expires_at / days_remaining), is_paid_active() with
  admin-bearer-token bypass, and a require_paid FastAPI dependency that
  raises 402 Payment Required for free-tier callers.
- POST /api/analyze (portfolio AI commentary) gated behind require_paid.
- Settings page surfaces credit window when active ("free · credit · N
  day(s) remaining (expires YYYY-MM-DD)") and the upgrade hint when not.
- Admin CLI: python -m app.cli {grant-credit,revoke-credit,show-status}.
  grant-credit is idempotent — extends from max(now, current expiry) so
  re-running the command never erodes an existing grant.

Migrations 0013 (referrals) and 0014 (credit_until). Tests cover the
paid-status truth table, code generation + normalisation, CLI argument
parsing, and the pending-cookie ref roundtrip (29 new tests).

alembic/versions/0014_user_credit_until.py

@@ -0,0 +1,36 @@
+"""users.credit_until: timestamp until which a free-tier user has paid-tier
+access. Set by:
+
+  - Admin CLI (`python -m app.cli grant-credit <email> <months>`) — manual
+    grants for testing & goodwill, in lieu of Paddle in Phase D.2.
+  - Paddle webhook (Phase D.3) — referral conversion bumps both parties'
+    credit forward by 3 months at 50% off.
+
+Null means "no credit". The `is_paid_active` helper in app/services/access.py
+treats `credit_until > now()` as paid-equivalent.
+
+Revision ID: 0014
+Revises: 0013
+Create Date: 2026-05-21
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+
+revision: str = "0014"
+down_revision: Union[str, None] = "0013"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "users",
+        sa.Column("credit_until", sa.DateTime(timezone=True), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("users", "credit_until")